The methods for driving OLED can be divided into passive matrix OLED (PMOLED) and active matrix OLED (AMOLED). The AMOLED uses thin-film transistors (TFTs) and capacitors to store signals for controlling the brightness and gray scale of the OLED. Although the cost and technical threshold for fabrication of the PMOLED are lower, the products of PMOLED are still limited to about 5 inches in size and the resolution cannot increase due to the constraint of the driving method. Thus they are restricted in the market of low resolution and small dimension. To achieve a higher resolution and a larger screen, active driving method must be used. The active driving method uses capacitors to store signals, so that the pixel can still maintain the original brightness after the scan line scans it. In the passive driving, only the pixel that is selected by the scan line will be lighted. Thus under the active driving method, OLED does not need to be driven to a very great brightness. As a result, it has a longer service life and can achieve a higher resolution. To couple OLED with TFT technology makes active driving of OLED possible, and meets the market demands for the smoothness of display and ever-higher resolution.
The technologies for growing TFT on the glass substrate can be amorphous silicon (a-Si) process and low temperature poly-silicon (LTPS) process. The main differences between LTPS TFT and a-Si TFT are in electricity and manufacturing complexity. LTPS TFT has a higher carrier-mobility which means that TFT can better provide sufficient current, but its manufacturing process is more complicated. By contrast, a-Si TFT has a lower carrier mobility than LTPS, but its manufacturing process is simpler and well developed, and therefore a-Si TFT has a better competitiveness in terms of cost.
Because of the constraints in manufacturing process of LTPS, the TFT elements being fabricated have variations in threshold voltage and electron mobility. As a result, each TFT element has different characteristics. When the driving system adopts analog voltage-modulation to display gray level, even if the input data-voltages are the same, the TFTs generate different output currents such that the OLEDs of different pixels on the display panel will display different brightness due to different characteristics of TFT for different pixels. This phenomenon causes the ill gray level on OLED display panel and severely damages image uniformity of the panel.
To remedy the shortcoming of uneven image uniformity mentioned above, U.S. Pat. No. 6,229,506, entitled “Active Matrix Light Emitting Diode Pixel Structure and Concomitant Method” discloses a pixel circuit that includes 4T2C (4 TFT transistors and 2 capacitor) as shown in FIG. 3. It has an auto-zero mechanism to compensate threshold voltage variations of the TFT elements to improve the image uniformity. Its operating principle is as follows:
The driving time sequence of control signals of the driving circuit is divided in auto-zero phase 410, load data phase 420 and illuminate phase 430. Refer to FIG. 4 for the control signal time sequence based on FIG. 3.
Before entering the auto-zero phase 410, transistor P3 and transistor P4 are OFF, and transistor P2 is ON. In the meantime, current flowing through Organic Light Emitting Diode (OLED) 360 is the current of a preceding frame, and this current is controlled by Vsg of transistor P1 (voltage difference between the source and gate, i.e. the voltage difference between two ends of capacitor element Cs′).
After having entered the auto-zero phase 410, transistor P4 is initially ON, and transistor P3 is ON as follow in order to connect the drain and gate of the transistor P1 to form a diode connection. Then transistor P2 is OFF, and the voltage of the gate of transistor P1 will increase to a voltage value which is equal to the high potential (Vdd) subtracts the threshold voltage (Vth) of transistor P1, i.e. the voltage difference between two ends of the capacitor element Cs′ is the threshold voltage of transistor P1. Then transistor P3 is OFF, and the threshold voltage (Vth) of transistor P1 is stored in the capacitor element Cs′ to fulfill the auto-zero phase operation.
When entering the load data phase 420, if voltage variation on the data line 310 is ΔV, and is connected to the gate of transistor P1 through transistor P4 and capacitor element Cc′, the voltage difference between two ends of the capacitor Cs′ will be ΔV×[Cc′/(Cc′+Cs′)] plus Vth originally stored in Cs′, i.e. Vsg of transistor P1 will include Vth of transistor P1. Thus current output from transistor P1 relates only to voltage variation ΔV on the data line 310 without being affected by Vth of the transistor P1 in each pixel.
Finally, entering the illuminate phase 430. Transistor P4 is OFF, and transistor P2 is ON. Transistor P1 will output current of the present frame flowing through OLED 360 to enable OLED 360 element to illuminate.
Although the pixel circuit of 4T2C can compensate variations of the threshold voltage (Vth) of the transistors in each pixel and improve image uniformity of the entire display image, the elements being used include four transistors and two capacitors. As the capacitors take a lot of area in the pixel, aperture ratio of the pixel will decrease significantly. Moreover, in addition to the data line 310, scan line 320 and supply line (Vdd) 350, it also requires control circuits such as auto-zero line 330 and illuminate line 340. The driving method becomes very complicated. Hence it requires non-standard scan driving IC and data driving IC, and fabrication cost is higher.